The present invention relates to an apparatus for preheating, by using a suspension preheater, a granular material and or including powder material (called simply as granular material) such as limestone, dolomite or the like.
A material such as limestone, dolomite or the like is preheated in a suspension preheater having a plurality of cyclones arranged to form multiple stages, by heat which is derived from gases wasted from a calcination furnace. The preheated material is then introduced into and treated in the calcination furnace spas to become lime or magnesium oxide, through a reaction expressed by the following formula (1) or (2): EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2 ( 1) EQU MgCO.sub.3 .fwdarw.MgO+CO.sub.2 ( 2)
In general, in a suspension preheater, calcined granules accompanying the waste gas and carbon dioxide gas contained in the waste gas make a re-carbonating reaction as expressed by the following formula (3) or (4) at a temperature ranging between about 600.degree. C. and 800.degree. C.: EQU CaO+CO.sub.2 .fwdarw.CaCO.sub.3 ( 3) EQU MgO+CO.sub.2 .fwdarw.MgCO.sub.3 .fwdarw.( 4)
Limestone or magnesium carbonate generated as a result of the re-carbonating reaction has been finely pulverized, so that deposition of hard matters occurs on the inner walls of the cyclones constituting the suspension preheater in the above-mentioned temperature range. The deposition of hard, matters, referred to also as "coating" grows heavy the longer the suspension preheater operates, thus hampering continuous operation of the suspension preheater, as known to those skilled in the art.
Another problem is that proper exchange of heat between the material and, the waste gas cannot be made because the period of residence of the material in the duct of the suspension preheater is too short. As a consequence, the temperature of the waste gas is raised to increase the heat consumption.
As a measure for overcoming the above-described problems, a method has been proposed in Japanese Patent Laid-Open Publication No. 53-110624 in which the duct is restricted at its intermediate portion so as to realize a jet layer structure in which the waste gas and the material form mutually contacting counter flows, aiming at increasing the period of stay and improving the heat exchanging efficiency. This preheating measure, however, is still unsatisfactory. Namely, although a restriction is provided in the duct to increase the period of stay of the material, the material can stay in the duct only for an extremely short period because the velocity of the waste gas has to be 35 m/s to 40 m/s in order to blow the material particles of a particle size around 4 mm. Thus, the exchange of heat between the material and the waste gas is insufficient and the problem of large heat consumption still remains unsolved. Furthermore, supply of the material at an excessively large rate into a cyclone from a cyclone just upstream thereof may allow the material to directly reach the downstream cyclone, with the result that the quality of the product is deteriorated due to mixing of raw material.
Proposals are also made for the purpose of obviating the above-described problems of the prior art. For instance, Japanese Utility Model Publication No. 63-1196 discloses an apparatus in which cyclones of the stage or stages which develop temperatures between 600 and 800.degree. C. where the re-carbonation reaction takes place are arranged in a plurality of parallel lines which are alternately used to enable a continuous operation of the suspension preheater. Japanese Utility Model Laid-Open No. 2-63544 also discloses an apparatus in which a plurality of parallel cyclones are used in a switching manner. In this type of apparatus, the temperature of waste gas tends to be elevated due to, for example, a rise in the calcination temperature in the calcination furnace and insufficiency of the heat exchange conducted in the suspension preheater. Consequently, the aforementioned coating due to re-carbonation takes place not only in the parallel cyclones which designed to operate at the aforesaid temperature range but also in the cyclones which are on the upstream side of these parallel cyclones. Consequently, the preheating apparatus requires renewal or switching of these upstream cyclones, failing to meet the aforesaid requirement for long continuous operation.